1. Field of the Invention
The present invention relates to a color conversion relation derivation method, a color conversion relation derivation apparatus, and a color conversion relation derivation program storage medium, which derive a color conversion relation between a first color space and a second color space.
2. Description of the Related Art
There are known various types of input devices for driving image data upon receipt of an image, for example, a color scanner for deriving image data upon reading of a recorded image, and a digital still camera (DSC) for deriving image data by means of reading through formation of an image of the subject on a solid state imaging device. According to those input devices, image data is represented by data of a range of for example 0 to 255 on each of three colors for example of R (red), G (green) and B (blue). This range represents a range of colors, which can be dealt with as hardware in the input device. However, this range of colors falls short of the whole color spaces capable of being recognized by human. For this reason, even if a color of the original image involves extremely plentiful expression, once conversion into image data by the input device may restrict the image represented by the image data to colors within the color expression area represented by a cube and a rectangular parallelepiped within R, G, B color spaces, which can be dealt with in the input device.
With respect to an output device for outputting an image in accordance with image data, there are known various types of output devices, for example, a photographic printer for recording an image on a photographic paper in such a manner that the photographic paper is exposed with a laser light and the exposed photographic paper is developed, a printer for recording an image on a paper in accordance with an electrophotographic system and an ink jet system, a printing machine for creating a large amount of printed matters through running a rotary press, and an image display unit for displaying an image on a display screen in accordance with image data, for example, a CRT display and a plasma display. Also with respect to those output devices, in a similar fashion to that of the above-mentioned input device, there are color representation areas according to the respective output devices. That is, the output devices can represent various colors in accordance with image data representative of three colors for example, R, G and B, and image data representative of four colors of C (cyan), M (magenta), Y (yellow), and K (black). However, the range of colors, which can be dealt with as hardware in the output device, is restricted to colors within the color representation area (for example, a color representation area represented by numeral values of 0 to 255 on each of three colors of R, G and B) represented by a cube and a rectangular parallelepiped of output device color spaces (for example, RGB color spaces, and CMYK color spaces).
Even if certain image data (for example, image data representative of (R, G, B)=(50, 100, 200)) is concerned, a color of an image, which will be obtained in accordance with the image data, is varied depending on a sort of the output device. In this respect, the same matter is applicable to a relation between the input device and the output device. Even if a certain output device outputs an image in accordance with image data (R, G, B)=(50, 100, 200) obtained through a certain input device, there is no general consistency between a color of an original image of the input device and a color of an output image of the output device. Accordingly, when it is intended that a certain input device is used to read an image so that image data is obtained, and a certain output device is used to reproduce the original image in accordance with the image data, there is a need to convert the image data between the input device and the output device, but not directly transmitting the image data obtained in the input device to the output device. Here, the conversion is performed taking a color of an image into consideration, and the conversion of the image data is referred to as a color conversion. LUT (Look-up table) defining a mutual relationship of image data by the color conversion is referred to as a color profile. In other words, the color profile defines a conversion relationship of the color space coordinate values.
As mentioned above, even if image data, which are the same as one another in numeral value, are concerned, colors to be represented in respective devices are mutually different. Recently, however, it is desired to construct a device independent type of system capable of incorporating therein a plurality of sorts of input devices and a plurality of sorts of output devices, in which the input devices and the output devices are connected with one another. According to such a device independent type of system, to perform a color conversion among the devices, there is adopted a technology of applying a color conversion in which a common color space independent of a device (a space of device independent data), for example, L*a*b* color space, is interposed, and image data on the color space depending on the input device, which is obtained in the input device, is converted into the common color space, and a color conversion of converting image data on the common color space is converted into image data on the color space depending on the output device. According to the device independent type of system, as the above-mentioned color profile, there are created an input profile defining a color conversion from the color space depending on the input device to the common color space, and an output profile defining a color conversion from the color space depending on the output device to the common color space (cf. for example, Japanese Patent Application Laid Open Gazette TokuKaiSho. 60-105376, Japanese Patent Application Laid Open Gazette TokuKaiSho. 61-288662, and Japanese Patent Application Laid Open Gazette TokuKaiHei. 4-196675).
The color conversion using such a color profile is carried out on operational basis on a device having an arithmetic operating function or a computer connected to a device having no arithmetic operating function. Of the color profiles, there is one defining a function representative of a conversion relation of one direction (a forward direction) toward from a certain color space to another color space. With respect to the reverse color conversion, in many cases, this conversion is performed by means of reversely referring to the function representative of the conversion relation of the forward direction. But it is desired that the accuracy of the reverse reference is greater accuracy.
By the way, in order to implement a color conversion with greater accuracy, there is a need to provide a smoothing wherein a color conversion function is provided by a smooth curve. And thus there are proposed a method using a polynomial approximation (for example, cf. Japanese Patent Application Laid Open Gazette TokuKai. 2004-54318), a method using a smoothing spatial filter, and a method using a smoothing spline.
However, according to the method using a polynomial approximation, the approximation is made with one polynomial and thus the color conversion function is easy to be of expression of high order. Accordingly, the use of the color conversion function for color conversion may take a very lot of operating time. Generally, a color reproduction area of a color space (here an input RGB color space) depending on an input device is not coincident with a color reproduction area of a color space (here an output RGB color space) depending on an output device. In case of two stages of color conversion, for example, the input RGB color space—a common color space (here L*a*b* color space—the output RGB color space, if the color conversion function representative of the later stage of color conversion is of expression of high degree, it is difficult to ensure accuracy of the later stage of color conversion on a portion going over the color reproduction area at the output side, of the color reproduction area at the input side. The method using a smoothing spatial filter is a simple method. However, according to this method, filtering up to obtaining a sufficiently smooth curve may enhance a discrepancy from the original value. Accordingly, this method brings about a problem the accuracy of the color conversion is rather lowered. The method using a smoothing spline is of high accuracy, but be easy to take a lot of time for arithmetic operation.
As to accuracy of the color conversion, the larger number of sets of arbitrary sample points provided on one color space and points of another color space associated with the sample points is prepared, the greater accuracy can be expected. But the larger number of sets is prepared, the more lot of time for arithmetic operation it takes.